THE LEADERS of the United States and the Soviet Union are moving rapidly to an agreement on nuclear forces in Europe that is plainly absurd -- absurd, that is, if we believe NATO assumptions about Soviet aggressive intentions. The evidence suggests, however, that it is not the potential agreement that is faulty but the NATO assumptions -- specifically that the Soviets are likely to invade Europe. These assumptions remain grounded in 1948-1953 thinking on the nature of the danger we face, while Soviet thinking has evolved.

Throughout most of the post-World War II period, the Soviets had always assumed that major conflict with the United States would involve world war. In the 1979-82 period, however, there emerged the concrete possibility of major conflict with the U.S. in the area north of the Persian Gulf. Soviet leadership had then to decide whether or not such conflict could be contained. If escalation to world war was inevitable, then Soviet forces were already poised and, at the appropriate moment, would launch an offensive into Western Europe. If, however, escalation was not inevitable, then there would be no offensive into Europe, and the Soviets would need only to hold in the west while engaging U.S. forces to their south.

Evidence is mounting that the Soviets decided that such a conflict might be contained. There is the new structure of Soviet theater commands, the new emphasis in their military writing on strategic defensive operations. There is also their new approach to conventional arms control including proposals for sweeping reductions in the two sides' forces. This change in doctrine implies more than the possibility of the Soviets' adopting a defensive posture in Europe. For some 70 years, a central element of Russian defense policy has been the requirement to be able to mount a continental-scale offensive to the west. By downgrading this enduring strategic imperative, they have removed a lynchpin from the framework of Soviet strategic thinking and allowed political and economic factors to assume a new salience.

Soviet statements now play down the differences between the capitalist and socialist systems and deemphasize the military aspects of national security in favor of the political. They assert that nations are now interdependent, that national security depends on mutual security; that the arms race has reached a new and dangerous stage; and that there would be no victors in a conventional world war, while nuclear war would destroy mankind. The Soviets are even talking of military "sufficiency" rather than parity.

There is nothing very original about these ideas -- most were propounded long ago in the West. What is radical is that the Soviet leadership appears to be taking them seriously. We are faced with a new Soviet "line" and the evidence argues that this is not just a tactical adjustment, but a genuine new direction in Soviet defense and foreign policy.

How should the West respond to the development? In principle, we should be delighted. For forty years we have blamed most of our international problems on the communist view of a divided world, on the Russian sense of insecurity, and on the Soviet policy of planning to win a world war should it prove unavoidable. In practice, however, the Western reaction has been mixed and the official response muted and muddled. To understand why, we must go back forty years.

Between 1948 and 1953, the two sides' threat perceptions were mirror images, both understandable but neither fully justified. The West feared a communist takeover of Europe, using political subversion and Soviet ground forces. The Soviets faced the U.S. atomic monopoly. They feared a preventive attack on their nuclear development facilities, the "roll back" of their presence in Eastern Europe, and a capitalist attempt to unseat the communists from power in Moscow. Both sides feared war.

Following Stalin's death, Soviet doctrine and perceptions of threat evolved. In 1956 it was decreed that war between the two social systems was no longer fatalistically inevitable. By 1960, the likelihood of a premeditated attack by the West had been discounted, but the danger of world war remained inherent in the nature of international relations. In the late 1960s it was ruled that it was not inevitable that a world war would be nuclear and involve massive strikes on Russia. And in the early 1980s, as described earlier, it was decided that a major conflict with the United States outside Europe might be contained rather than inevitably escalating to world war.

This process of doctrinal evolution progressively relaxed the military requirements for Soviet national security. It enabled the emergence of "new political thinking" that emphasized the non-military aspects of security and stressed the dangers of the present situation.

Western military thinking has not evolved in the same way and official perceptions of the Soviet threat remain frozen in the mold of 1948-53. This is partly due to the problem of reaching agreement in a democratic coalition, but the main reason is the concept of deterrence that has formed the core of Western defense policy since the early 1950s.

Central to a policy of deterrence is the assumption that one's opponent is tempted to take the action being deterred -- in this case to expand into Western Europe. A Soviet urge for aggression became a cornerstone of Western policy, an assumption that dispensed with the need to consider how such action would further Soviet interests, or to evaluate Soviet intentions against the evidence of their behavior. Soviet military capabilities and the posture of their forces facing Europe was sufficient proof of an expansionist urge. We ignored the fact that this evidence was also compatible with the need to take over Western Europe in the event of war provoked by other factors, if the Soviets were to avoid ultimate defeat. Yet the distinction is vital.

In one case the threat to NATO is Soviet aggression which, if not deterred, will lead to war. In the other case the threat is war which, if not avoided, will lead to a Soviet invasion. The appropriate response to one set of threat perceptions is wholly inappropriate to the other. This was demonstrated when NATO adopted "flexible response" in 1967, a concept intended to enhance deterrence. Instead, it prompted the Soviet conventional buildup in the 1970s that so alarmed NATO. Similarly, the 1983 deployment of Pershing II and cruise missiles to Europe was meant to reinforce deterrence, butmany in the West saw it as increasing the danger of war.

Of course, Western ambivalence about arms control has other roots. America acquired strategic superiority in World War II and has been understandably reluctant to give it up. America has consistently rejected arms limitations in areas where advanced technology or assymetries in force structure give it an advantage. The Strategic Defense Initiative (SDI) is the latest variation on this theme.

But contradictory perceptions of threat account for much of NATO's muddled response to Gorbachev's arms control initiatives. If the Soviets are set on military expansion, then arms control negotiations play into their hands, as Hitler taught us in the 1930s; but if the danger stems from the military capabilities ranged on either side, then arms limitations and reductions offer a way out of our predicament.

Michael MccGwire, a senior fellow at The Brookings Institution is author of "Military Objectives in Soviet Foreign Policy."